Communication networks are made up of a plurality of switching offices which serve the customer stations. These switching offices are interconnected by interoffice trunks to permit any customer station served by any office in the network to be connected to any other station served by any other office.
The amount of traffic, i.e., calls between offices, determines the number and type of interoffice trunking facilities. If the distance between offices and the amount of traffic warrants it, the individual trunks may be multiplexed on a common transmission media thus eliminating separate metallic talking paths between the offices for each trunk circuit. In one such system twelve separate trunk circuits are frequency multiplexed on a common analog carrier system between two switching offices. A savings in cable pairs is realized at the risk of a multiplicity of trunk outages if the carrier system should fail.
Recognizing the serious nature of carrier transmission system failures, system designers have taken steps to detect and report carrier failures and thereby minimize trunk group outages.
For example, the carrier terminals at each end of the carrier system have been provided with alarm arrangements to alert maintenance personnel if a carrier system fails. In addition, if the switching office at the carrier terminal is one of the more modern types having a stored program controlled processor, the processor can scan the carrier alarm circuit to detect the carrier failure. Having detected the failure, the processor can then automatically busy the trunks associated with the failed system.
While these arrangements are generally suited for their intended purposes, they lack certain features found in the present invention. For example, carrier system alarms generally use special purpose scan points. As system maintenance becomes more complex, scan points become scarce and more costly. Also with most prior art carrier failure alarm arrangements the alarm circuit itself could fail and be unable to report a legitimate carrier system failure.
Accordingly, a need exists for a more economical and continuously self-testing method of detecting carrier transmission system failures.